**3. Anti-QS approaches**

Because of widespread heightened antimicrobial resistance, the conventional means of treating bacterial infection, antibiotic therapy, is now increasingly impractical, such that alternative approaches are being considered [95]. The presence of biofilm, efflux pumps, and persister cells each exacerbate drug resistance [96]. Targeting QS by disturbing cell-cell communication is a way to combat biofilm [97]. Moreover, the effectiveness of different potential inhibitors against QS has been reported [98]. Various strategies are proposed to disrupt QS, including receptor inactivation, signal inhibition (by natural or synthetic inhibitors), signal degradation by quorum quenching enzymes, blocking QS by antibodies, and applying antibiotics as a cotreatment [98, 99].

Targeting AIPs is a good way of treating QS and considerable effort has been made to date to find inhibitors [100]. A known approach suggested in this context is to cope with RNAIII, due to its key role in QS. Reportedly, RNAIII inhibitory peptides (RIPs) have shown inhibitory effects on *agr* and biofilm. It is believed that targeting this molecule will diminish the production of some virulence factors and toxins [101, 102]. Similarly, based on the inhibition of *agr* of another subgroup [103], natural and synthetic AIPs may be introduced as potential inhibitors. Different inhibitors include nonpeptidic (P3 inhibitor) and synthetic molecules, cyclic dipeptides (from *Lactobacillus*), ambuic acid (a fungal extract), licochalcone A (LicA, a plant extract), antivirulence agents such as naphthalene and biaryl compounds, organic compounds (by interfering with *agr*-DNA binding), and savirin (*S. aureus* virulence inhibitor). Each of these actively inhibits QS in *S. aureus* [104–110]. Monoclonal antibodies, applied as both passive and active immunotherapeutic regimens, have also yielded promising results [49]. Collectively, many approaches that target *agr* and AIPs have been tested. Targeting AIPs via extracellular therapy is advantageous over targeting *agr*, as complications of intercellular therapy such as degradation do not arise.

Another treatment approach for Gram-negative bacteria is based on phenolic compounds. When tested extensively against AHL QS in *P. aeruginosa*, the novel phenolic derivative GM-50 reduces biofilm-related virulence, thereby enhancing antibiotic efficacy [111]. In addition, food-associated bacteria such as lactobacilli can exploit antibiofilm activity by interfering with AHL QS [112]. The efficacy of probiotics against QS has been indicated in previous studies. Presumably, they exert their effects via secretion of metabolites and microencapsulation [113, 114].

Targeting AI-2 lessens the pathogenicity of different bacterial species [115]. Various natural products such as D-galactose and furanocoumarin (reducing AI-2 synthesis), apigenin, hexadecenoic acid, and citral have shown promise at inhibiting *V. harveyi* QS [116–120]. In terms of chemicals, halogenated furanones are effective

against AHL and AI-2, subsequently affecting biofilm formation [121]. In *C. jejuni*, two fatty acids, decanoic acid and lauric acid, were found to be useful against AI-2 at 100 ppm (preventing 90% of AI-2 activity). As a result, biofilm formation and motility of the bacterium were reduced substantially [115]. Similarly, different naturally occurring compounds including monoterpenoid glycosides, emodin, and antimicrobial peptides showed satisfactory inhibition of LuxS/AI-2 in *Streptococcus suis* [122–125].

Currently, there is no drug approved for clinical use, although research and development efforts are continuously making progress toward this goal. As a consequence of administering anti-QS drugs, bacterial virulence (selective pressure will result in no further negative implications) applied should decrease, which is of great importance when seeking novel, effective treatments [111, 126].

#### **4. Conclusions**

The complex adaptive regulatory system of QS stands out as the most pivotal mechanism of pathogenicity exhibited by bacteria [127]. Regarding therapy, because of the emergence and widespread prevalence of antibiotic resistance, cotreatment with alternatives as well as surgical removal of infected tissue surrounding implanted medical devices, is being increasingly used. Quenching and inhibitory substances suppress the virulence and pathogenicity of those bacterial pathogens that use QS. Because QS has a critical role in many physiological behaviors such as biofilm formation, exoenzyme secretion, siderophore functioning, membrane vesicle formation, swarming, and sporulation, QQ is becoming a popular strategy [128]. Thus, an in-depth knowledge of biofilm, sensitive antibiotics, penetration, and anti-QS agents will help to inform antimicrobial therapies to overcome biofilm infection [129].

Multiple activities of anti-QS agents have been identified, for instance, QS receptor inactivation, QS signal inhibition, degradation of QS signals, and antibodies to block QS, as well as combination therapies such as flavonoids or immucillin A in *P. aeruginosa,* lactonase in *Acinetobacter baumanni*, AP4-24H11 in *S. aureus,* and farensol with ß-lactamase antibiotics in *S. aureus* [130–134]. Given this premise, a QS inhibitor can modulate gene regulation via either of two strategies: interposition with signal generation and signal reception [135, 136]. Notably, many QS inhibitors, such as furanones and halogenated and acylated furan structures, are improved by competing with the AHL pheromone in *P. aeruginosa* [137, 138]. Furthermore, RIPs have shown promise against *S. aureus* [139].

Negative aspects of disturbing the QS system should be considered. Inadvertent or unregulated modulation of microbiota through the use of QS quenching compounds or inhibitors may cause a disequilibrium of normal microflora. This concept developed as AI-2 molecules resemble bacterial presence to provide microflora [128, 140]. At the same time, pathogenicity tends to increase by applying quenching agents that may contribute to the long-term survival of *S. aureus* [141–144]. In particular, staphylococcal QS *agr* mutant strains tend to develop persister forms as well as raised biofilm production [143, 145]. A possible strategy is to apply QS quenching only in the absence of biofilm. This stems from the observation of applying selective pressure to preserve *agr* as a planktonic form rather than in biofilm [146].

An important clinical consideration is to determine the strain susceptibility and optimal form of treatment, otherwise, the patient's condition may worsen [102]. In addition, limitations and challenges should be carefully weighed. For example, in *Quorum Sensing in Biofilm DOI: http://dx.doi.org/10.5772/intechopen.113338*

*S. aureus,* the type of condition should be considered, as *agr* performs contrary roles in biofilm and chronic infection. Of note, most studies on QS drugs have been carried out using a single laboratory strain. Although such research models provide valuable information, it is challenging to extrapolate with confidence to clinical settings in, for instance, the case of AIPs in *S. aureus,* as species subgroups are identified. Finally, the selectivity and safety of QS inhibitors, while minimizing disturbance of microflora, are important factors for human usage. Designing a library of QS inhibitors and determining their IC50 values is a suggested area for future research.
